An apparatus that irradiates an object with light to detect information such as the shape of the object, or to induce change in the object has been used conventionally, such as, for example, a laser microscope, an optical pickup device, a laser processing machine, etc. In such an apparatus, in order to achieve higher resolution, it is desirable that the diameter of the beam spot irradiated from a light source and focused onto an object be as small as possible. In general, the minimum size of the beam spot is determined by the diffraction limit, and is proportional to the wavelength of light. Thus, the shorter the wavelength of light irradiated from a light source, the smaller is the diameter of the beam spot.
However, a light source that emits light of shorter wavelength, for example, a laser that emits violet or ultra-violet light is generally higher in price than a light source that emits light of longer wavelength, for example, green or red light. Also, in the wavelength from violet to ultra-violet range, light transmittance of an optical material becomes generally lower as wavelength become shorter. Thus, availability of optical material having high light transmittance for violet or ultra-violet is very limited.
In this respect, radial polarization has been attracting increasing attention. Radial polarization is a polarization state in which direction of a polarization plane of linear polarization light is distributed symmetrically in a radial direction with the optical axis as the center. It is reported that, by focusing radial polarization with an objective lens, a light beam focused on the focal plane becomes z-polarized light (i.e., polarized light in which direction of electric field is same as the direction of propagation of light), and the light beam can be focused with a spot size smaller than the spot size of X- or Y-polarized light determined by the diffraction limit, and thus, it is possible to achieve a super resolution effect. A microscope apparatus which incorporates a light modulator element for converting linear polarization light from a light source to radial polarization light has been proposed (see, for example, Japanese Unexamined Patent Publication No. 2010-19630 and Japanese Unexamined Patent Publication No. 2010-15877).
A method has been proposed for correcting wave front aberration by disposing a liquid crystal light modulator element in an optical system to impart phase distribution to the light beam passing through the light modulator element so as to cancel wave front aberration produced in the optical system (see, for example, Japanese Unexamined Patent Publication No. 2005-202323).